feat: shared local-stats pattern package for Floke apps

[krwg]

Summary

BLIP already has achievements / session-stats; Cultiva has habit-analytics. Extract the architectural pattern (append-only events → periodic aggregation → cached summary), not a 1:1 domain merge, into an internal package (e.g. @krwg/local-stats) so the next Floke app does not reinvent offline local analytics a third time.

Scope

• Package skeleton + documented pattern
• Optional thin adapter notes for BLIP session-stats and/or Cultiva analytics (migration can be incremental)

Acceptance

• Package/repo or workspace module exists with README of the pattern
• At least one consumer path sketched or partially wired
• Remains offline / no telemetry